Automatic volume control for amplifiers



Dec. 27, wasp LTHOMPSON 2,141,944

AUTOMATIC VOLUME CONTROL FOR AMPLIFIERS Filed Au 4, was

Fig.1.

' T0 LOUD SPEAKER AMPLIFIER OUTPUT 32/} 3? INPUT RECTIFIER DIODE FiLTE%4i INVENTOR Liumln Thompson ATTORNEY 4-44 Patented Dec. 27, 1938AUTOMATIC VOLUME CONTROL FOR AMPLIFIERS Lincoln Thompson, Cheshire,Conn.

Application August 4, 1936, Serial No. 94,215 13 Claims. (Cl. 179-171)This invention relates to alternating current amplifying apparatus, andmore particularly to an automatic gain control system for use with audiofrequency currents.

One object of this invention is to provide an improved system formaintaining a uniform output of a multi-stage Vacuum tube amplifier inwhich a portion of the amplified output of the amplifier is rectifiedand the direct current resulting therefrom is utilized to vary theresistance of the plate-filament circuit of an early low-energy stage ofthe amplifier in such a manner as to compensate for variations instrength of the input signals.

A further object is to provide an improved system of the above nature inwhich a diode is utilized for automatically controlling theamplification of the first stage of the amplifier.

A further object is to provide an automatic gain control system of theabove nature in which use is made of a pair of diode tubes in push-pullrelationship for introducing resistance into the plate-filament circuitsof a pair of triode tubes in push-pull relationship.

A further object is to provide a system of the above nature forautomatically monitoring the intensity of the output signals of a vacuumtube amplifier by rectifying a portion of said output signals andcausing the higher direct current rectified potentials which result fromthe stronger output signals to produce a greater shunting action on theplate-filament circuit of the diode control tubes than the lowerpotentials resulting from the weaker signals thereof.

A further object is to provide an automatic, control system of the abovenature employing means to produce a definite time lag so as to preventthe possibility of distortion in the amplified signals.

A further object is to provide an automatic volume control system of theabove nature which will be simple in construction, inexpensive tomanufacture, easy to install and manipulate, compact, and very efficientand durable in use.

With these and other objects in view, there have been illustrated on theaccompanying drawing two forms in which the invention may beconveniently embodied in practice.

In the drawing:

Fig. 1 represents a general diagrammatic view of an amplifier systemembodying the invention.

Fig. 2 is a detailed circuit diagram of the same.

Fig. 3 is a similar circuit diagram of a modified form of the invention.

In previous systems for controlling the volume of multi-stage vacuumtube amplifiers, it has been customary to rectify a part of theamplifier output to produce a direct current potential which isproportional to the signal strength. This potential was then filteredand applied to 5 the grid-filament circuit of one of the tubes in theamplifier for varying the grid bias of said tube in such a manner as tocompensate for any increase or decrease which may take place in thestrength of the signal.

It has been found, however, that such a control system could not be usedon the audiofrequency stages of the amplifiers without causingdistortion. This was due to the fact that the characteristic curves ofthe grid-filament circuits are not straight lines, but are curved lines,and hence, would produce distortion in the high and low strengthaudio-frequency signals applied thereto. This disadvantage wasespecially evident in the amplification of a two- 2. way telephoneconversation in which weak signals from a distant telephone transmitterare combined with strong. signals from a nearby transmitter.

According to the present invention, the above and other disadvantageshave been overcome and a system has been designed which willautomatically level the volume in the output circuit of an amplifier sothat both strong and weak signals entering at the input will leave theamplifier at approximately the, same degree of loudness.

The present invention contemplates applying a part of the amplifiedoutput current from a multi-stage amplifier to a rectifier as in theprevious systems, but instead of applying this rectified potential tothe grid-filament circuit of one of the amplifier tubes, it is appliedto a twoelement tube (diod) which is operated in such a manner that theresistance of the diode varies inversely as the potential appliedthereto. The diode tube is shunted across the plate-filament circuit ofthe first tube of the multi-stage amplifier so as to load it down anddecrease or increase the resistance of said plate-filament circuit,according to the variations in the signal strength. In other words, theamplification of the first amplifier tube is caused to vary inverselywith variations in strength of the input signals.

In the use of the improved control system herein disclosed, theoperation is not instantaneous,-a condition which might distort the waveform of the signals-but a timelag occurs which prevents such distortion.

Specifically, the improved, system is designed automatic volume 50 toimpress a portion of the amplified output current of a multi-stageamplifier upon a Vacuum tube rectifier preferably of the full wave type,and operated in the conventional manner for rectifying alternatingcurrents. The rectified output is then passed through an ordinaryinductance-capacity type of smoothing filter which is connected to adiode operated in such a manner that the filament-to-plate impedancevaries with the applied voltage. This impedance is included in the platecircuit of the first amplifier tube by means of an isolating condenser.Since the diode impedance can be made to vary from infiinity to lessthan one thousand ohms, a wide range of volume control is possible,particularly when thermionic tubes of high amplification constants areemployed.

In order to stabilize the operation of the amplifier and preventregeneration, the invention contemplates the employment of amplifiertubes and diodes arranged in pairs according'to the push-pull system.With this arrangement the smoothing efiected by the filter needs to befar less perfect than would otherwise be the case because the pulsesremaining in the smoothed diode plate voltage are applied in the samephase to the plates of the amplifier tubes which are 180 degrees out ofphase with respect to the amplified signals. Hence, any tendency towardregeneration is canceled out. Moreover, it is possible by increasing theamount of smoothing to adjust the time lag in the smoothing circuit.

Referring now to the drawing in which like reference characters denotecorresponding parts throughout the several views, the letter A indicatesa three-stage push-pull resistance-coupled amplifier. The first stagecontains a. pair of thermionic vacuum tubes l0 and H, across the platesof which are shunted a pair of diode tubes I2 and I3 which are adaptedto effect the volume control. The tubes l0 and II are resistancevcoupled in the conventional manner by means of resistances l4; l5, l6and IT, to a pairv of thermionic tubes l9 and 20 forming the secondstage of the amplifier. The-tubes l9 and 20 are in turnresistance-coupled by means of the resistances 2|, 22, 23 and 24, to apair of push-pull connected output thermionic tubes'26 and 21 of thethird stage of the amplifier. Resistances I8 and 25 serve to providenegative biasfor the tubes I9, 20 and 26, 21 in a conventional manner.The output of the tubes 26 and 21 is passed through a transformer 28consisting of a primary coil 29 located in the plate-filament circuitsof the tubes 26 and 21 and a secondary coil 30 connected to a loudspeaker 3|. The secondarycoil- 30 is also connected by a pair ofconductor wires 32 and 33 to a rectifier 34. The rectifier 34 includes acenter tap choke 35 which is also connected to the terminal plates ofthe choke triode tube 36. The center tap of the choke 35 providesthe'negative terminal of the direct current output of the rectifier inaccordance with conventional design, and is connected to a conduc- 65tor wire 31 leading to one terminal of a smoothmg filter 39 to bedescribed later. The cathode (or filament) of the rectifier 34 providesthe pos- 1t1ve terminal thereof and is connected by a con duo tor 38a tothe other terminal of the filter 39 0 which consists of a choke 4B and apair of condensers 4| and 42. The magnitude of the energy put of therectifier is controlled by a variable resistance 43. 5 F Order toProvide the .range of impedance desired for the Plate circuits of thetub s S an II, biasing potential is applied to the diodes l2 and I3 bymeans of a battery 44 and a variable potentiometer 45. The diodes I 2and I3 complete their circuits through a choke coil including a pair ofinductances 46 and 41 which are 5 shunted across the plates of the tubes[0 and II through a pair of isolating condensers 48 and. 49.

A battery 50 is employed to supply potential to the plates of each ofthe tubes I0 and H, 19 and 20, 26 and 21, and a battery 5.! energizesthe filaments of each of said tubes.

Operation In operation, input signals applied to the grid 16 circuits of'thetubes l0 and II will be amplified by these tubes in the usualmanner. Thus, the amplification of a tube may be found by multiplyingthe amplification constant by the impedance with which the plate circuitis loaded, and 20 dividing this product by the sum of this same loadingimpedance and the plate impedance of the tube itself.

It will be understood that if the impedance of the diodes i2 and I3 wereinfinite, the plate cir- 25 cuits of the tubes IB and H would be loadedonly with the plate resistances l4 and I5 in parallel with the gridleaks of the tubes l9 and 20, which are in series with the couplingcondensers 52 and 53. Energy will also flow through the parallel circuitcomprising the inductances 46 and 41 in series with the condensers 48and 49. The signals are then amplified in accordance with theseconditions and pass on to the tubes 1 l9 and 20, and thence to theoutput tubes 26 85 and 21.

When, however, the diode impedances are finite, it will be understoodthat the greater the output voltage entering the rectifier, the higherwill be the direct current rectified voltage ap-- 40 plied to the diodesl2 and I 3. Since the combined impedance of the diodes varies inverselywith the voltage over a limited range, the greater the voltage appliedthereto, the lower will be the diode impedances and the greater will bethe 45 shunting effect upon the tubes l0 and II. Consequently, theamplification of the tubes I0 and II will vary inversely as the outputvoltages applied to the diode.

If it is desired to have the control system oper- 50 ate only after thesignals have reached a certain predetermined level, the potentiometer 45may be adjusted to cause the diode impedances to be infinite until saidpredetermined signal voltage is reached, whereupon the bias will beovercome 55 and the impedances will be reduced to finite quantities.Since the diode characteristics are such that a rapid change ofimpedance occurs in response to the voltage applied thereto over alimited range, the accurate adjustment of the bias is quite important.

The adjustment of the resistance 43, which determines the proportion ofthe output signal which is to be applied to the rectifier 34, is alsoquite important because it permits the employment of the most usefulpart of the diode, characteristic curve for the range of the signalsencountered. The values' of the filter condensers 4| and 42, and theinductance 40 thereof, are also quite important in that they determinethe time lag or delay of the circuit, and thereby prevent distortion.

While the input energy has been herein described by the word signals, itwill be understood that said energy may be derived from any 7 otherdesired source of modulated audio or radio frequency currents, such as,the receiver of a land or radio telephone circuit, the output terminalsof a pickup device of an electric phonograph, the out-put terminals of amicrophone, the out-put terminals of a photo-electric circuit such asare used in sound motion pictures, etc.

It will be also understood that instead of operating to dampen or rendermore uniform the intensities of the output signals, the present systemmay be employed to accentuate or increase the contrast in intensity ofsuch signals within its general principles and scope.

In the modified form of the invention shown in Fig. 3 a control unit isdisclosed having two sets of diodes instead of one set as in the formshown in Fig. 2. This arrangement is of advantage in cases where a widerrange of control is desired as it permits the cooperative effect of thefour diodes 5d, 55, 56 and 57 to be adjusted, and thus enables the typeof time lag and other characteristics to be selected.

While there have been disclosed in this specification two forms in whichthe invention may be embodied, it is to be understood that these formsare shown for the purpose of illustration only, and that the inventionis not to be limited to the specific disclosures but may be modified andembodied in various other forms without departing from its spirit. Inshort, the invention includes all the modifications and embodimentscoming within the scope of the following claims.

Having thus fully described the invention, what is claimed as new, andfor which it is desired to secure Letters Patent is:

1. In a system for amplifying alternating currents, a vacuum tubeamplifier, means to apply alternating current to the input circuit of.said amplifier, a vacuum tube rectifier including a filtering impedancein the output circuit of said amplifier, a diode, means to apply aportion of the output current from said amplifier to said rectifier,thereby producing fluctuating rectified direct current, means to impresssaid rectified direct current upon said diode to produce a variableimpedance in said diode, and means for shunting said variable impedanceacross the plate circuit of said amplifier to control the gain thereof.

2. In a system for amplifying audio-frequency alternating currents, avacuum tube amplifier, means to apply alternating current to the inputcircuit of said amplifier, a rectifier including a filtering impedancein the output circuit of said amplifier, means to apply a portion of theoutput current from said amplifier to said rectifier, thereby producingoscillating rectified direct current, a variable impedance diode, andmeans to introduce the variable impedance of said diode into shunt withthe plate circuit of said amplifier, the impedance of said diode beinginversely proportional to the strength of said output current.

3. In a system for amplifying audio-frequency alternating currents, avacuum tube amplifier, means to apply alternating current to the inputcircuit of said amplifier, a vacuum tube rectifier including a filteringimpedance in the output circuit of said amplifier, means to apply aportion of the output current from said amplifier to said rectifier,thereby producing oscillating rectified direct current, and means tointroduce a variable impedance diode in shunt with the plate circuit ofsaid amplifier, the value of which is inversely proportional to thestrength of said output current.

4. In a system for amplifying at constant volume an audio-frequencyalternating current of variable power, a multi-stage vacuum tubeamplifier, a rectifier connected with the output circuit of saidamplifier, a diode in the output circuit of said rectifier, said diodebeing also included in the plate circuit of the vacuum tube of an earlystage of said amplifier, whereby the amplification of said tube willvary inversely to the strength of the output of said rectifier.

5. In a. system for amplifying "at constant volume an audio-frequencyalternating current of variable power, a multi-stage vacuum tubeamplifier, a rectifier connected with the output circuit of saidamplifier, a diode in the output circuit of said rectifier, said diodebeing also included in the plate circuit of the vacuum tube of an earlystage of said amplifier, whereby the amplification of said tube willvary inversely to the strength of the output of said rectifier, saidplate circuit also including an isolating condenser between the plateand said diode.

6. In a system for amplifying at constant vol- -ume an audio-frequencyalternating current of variable power, a multi-stage vacuum tubeamplifier, a rectifier in shunt with the output circuit of saidamplifier, a diode in the output circuit of said rectifier, said diodebeing also included in the plate circuit of the vacuum tube of an earlystage of said amplifier, whereby the amplification of said tube willvary inversely to the strength of the output of said rectifier, andmeans for controlling the potential bias between the anode and thecathode of said diode.

7. In a system for amplifying alternating currents, a vacuum tubeamplifier, a rectifier for a portion of the amplified energy of saidamplifier, a diode in the output circuit of said rectifier, said diodebeing also included in shunt with the plate circuit of said amplifier toautomatically produce a uniform volume of amplification.

8. In a system for amplifying alternating currents, a vacuum tubeamplifier, a rectifier for a portion of the amplified energy of saidamplifier, a diode in the output circuit of said rectifier, said diodebeing also included in shunt with the plate circuit of said amplifier toautomatically produce a uniform volume of amplification, andreactance-capacity means connected with said rectifier to produce a timelag in the oscillations passing therethrough.

9. In a system for amplifying alternating currents, a vacuum tubeamplifier, a rectifier for a portion of the amplified energy of saidamplifier, a diode in the output circuit of said rectifier, said diodebeing also included in shunt with the plate circuit of said amplifier toautomatically produce a uniform volume of amplification, andreactance-capacity means connected with said rectifier to produce a timelag of from .01 to 2 seconds in the oscillations passing therethrough.

10. In a system for amplifying audio frequency alternating currents, athermionic tube including a plate anode, a rectifier for the output ofsaid tube, a diode connected to the output of said rectifier, said diodebeing included in a circuit with said plate anode to cause theamplification of said tube to vary inversely with the input energy andthus maintain a uniform amplifier gain.

11. In an alternating current amplifier including an audion tube havinga grid, an anode and a cathode, means for suppressing fiuctuations inthe output circuit of said amplifiers due to variations in the strengthof the incoming signals, said means comprising a rectifier for a portionof the output of said amplifier, a diode connected to the output of saidrectifier, said diode being also included in the anode circuit of saidaudion tube to vary the shunting load thereof in accordance with thestrength of the output signals.

12. In an alternating current amplifier including an audion tube havinga grid, an anode and a cathode, means for suppressing fluctuations inthe output circuit of said amplifier due to variations in the strengthof the incoming signals, said means comprising a rectifier for a portionof the output of said amplifier, a diode connected to the output of saidrectifier, said diode being also included in the anode circuit of saidaudion tube to vary the shunting load thereof in accordance with thestrength of the output signals,

and a variable resistance in circuit with said rec-

